A safety ladder can comprise a vertical central ladder stringer and a plurality of horizontal rungs. The central ladder stringer can have a substantially C-shaped cross section with a front longitudinal slot, has a substantially planar rear transverse piece with rounded longitudinal edges. The rungs applied in the vicinity of the rear transverse piece from the outside to the central ladder stringer are attached thereto. The outer ends of the rungs are usually bent forwardly.
Safety ladders of these type are intended to protect a user from a possible fall and serious injury. For this purpose the user must wear a safety belt and harness having a releasable tethering device for attachment to the safety ladder, which is usually insertable in the longitudinal slot and is automatically guided and held on the central ladder stringer. The rungs of such a safety ladder can comprise single rungs, that is, rungs which extend alternately in opposite directions from the central ladder stringer, or twin rungs which extend in both directions.
In a known safety ladder of the above described kind (see to bulletin "FABA Steigschutz" (FABA Safety Ladder) of Fahrleitungsbau GmbH, D-4300, Essen 12, Germany, of October, 1983) the rungs are welded to the central ladder stringer on the rear transverse piece with the flat contacting surfaces in close contact with each other.
Welding is a comparatively expensive fastening means. Also the safety ladder requires a considerable amount of space during storage and transport to the site where it is to stand. Furthermore welding the rungs to the central ladder stringer at the place where the safety ladder is to stand is not feasible in practice. That is particularly true, when the safety ladder is not constructed of a stainless steel, or must be galvanized as a protection against corrosion.
In other safety ladders the rungs were fixed with the aid of bolts to the central ladder stringer. That is, however, because of the considerable number of rungs, also comparatively expensive.
Moreover the interior cross section of the central ladder stringer is unnecessarily reduced by the heads or nuts, so that the tethering device of the safety harness or belt must be constructed smaller or else the entire cross section of the central ladder stringer must be correspondingly enlarged.